This invention relates to a method and apparatus to detect and measure the quantity of particulate matter on a substrate, and more particularly to determine the quantity of powdered coating material on the surface of a moving metallic sheet without contacting the sheet or disturbing the coating.
Strips used to make metal containers and container ends for such beverages as beer, carbonated soft drinks, and the like are given coatings of resinous polymeric materials to form coherent, uniform, and functional coatings as thin as 0.05 mils in thickness. Such coatings can be formed from powdered coating materials, such as thermosetting epoxy powders, having average particle sizes in the range of from 1 to 15 microns, and preferably having an average particle size less than 10 microns. In applying such coatings, a preferable method is by the electrostatic deposition of such powdered coating materials. Among the advantages of the preferred electrostatic deposition method are efficiency in the use of the coating materials and the ability to achieve uniformity in very thin coatings. It is important, however, in such thin coatings that the requisite amount of powder necessary to provide a coherent protective coating on the substrate be present; thus, it is desirable that the presence and quantity of particulate material on the substrate be detected and measured in order to avoid the production of unusable sheet. This problem is particularly difficult in the commercial manufacture of such metallic sheet where it is coated as it is moved through a deposition zone at rates up to several hundred feet per minute. Any effort to contact the sheet will disturb the coating by dislodgement of particulate matter.